Methods of forming constituent layers, such as an organic layer, in production of organic electronic devices including an organic EL element, is loosely divided into dry processes by way of a deposition method etc., and wet processes by way of a coating method using a solution in which an organic material is dissolved in an organic solvent.
The dry process has an advantage that a uniform film can be formed to have a desired film thickness since an organic material and a metal are usually formed as films under a high vacuum of 10−4 to 10−6 Pa, substantially without ingress of moisture, oxygen, or impurities. Further, since the organic material, the metal oxide, and the metal can be formed as films continuously, it is easy to attain a high efficiency of an element and optimization of an element structure by providing each layer with discrete function. While, it has a problem in that uniform film forming in a large area is difficult, a material usage efficiency is low, and it is costly.
On the other hand, the wet process has a comparatively simple film forming process, is less costly, is of a large area, and allows flexible film forming, therefore attracts attention in recent years. Further, it is used in research and development of organic electronic devices, not only an organic EL element but also an organic transistor, an organic thin film solar cell, etc.
As examples of particular techniques, there may be mentioned coating methods including a spin coating method, a casting method, a spray method, etc., as well as immersing methods including a dip method, a self-organization method, the LB method, etc., and printing methods including an ink-jet method, a screen printing method, a roll-to-roll method, etc.
In the coating method by way of a spin coating method, film forming is performed to have a desired film thickness by dissolving an organic material in various solvents and controlling an amount of dropping and a concentration of the solution, a number of revolutions of a spin coater etc., in the atmosphere or under an inert gas atmosphere, such as in a glove box.
In such coating-type organic electronic devices, since a usual film forming material is soluble in an organic solvent, there is concern that a lower layer may be re-dissolved and mixed with an upper layer when stacking a coated film.
Therefore, in the organic EL element, for example a stacking method of using different solvents is employed in which polythiophene-polystyrene sulfonic acid (PEDOT:PSS) which is insoluble in an organic solvent and soluble in water is formed as a film on an ITO substrate, on which a luminescence layer is formed as a film by coating an organic solvent solution, containing an aromatic high-molecular material etc.
Further, it is often the case that an organic material used in a coating-type organic electronic device is unipolar, i.e., it has an electric charge transport property which allows transport of either holes or electrons. This follows that there is electric charge which passes to an electrode and does not contribute to electric charge recombination. Thus, there is a problem that such low carrier balance causes the organic electronic device to reduce its efficiency.
Furthermore, the electron injection layer in the coating-type organic electronic device conventionally employs Ba, Ca, etc. which are water-soluble or alcohol-soluble and are metals with lower work function are used and combined with Al, but they tend to be influenced by moisture or oxygen in the atmosphere, since such metals are very active.
Therefore, in order to attain a high efficiency of the coating-type organic electronic device, there is a need for an electron injection layer or an electron transport layer that can prevent the electric charge from passing through due to its stack structure, and is stable and applicable in the atmosphere.
Then, as for the alcohol-soluble electron injection material or electron transport material, the present inventors have paid attention to cesium carbonate (Cs2CO3), alkaline metal salts including lithium phenolate salts, such as sodium 8-quinolinolate (hereinafter abbreviated to Naq) shown in the following (Chemical Formula 1), lithium 8-quinolinolate (hereinafter abbreviated to Liq), lithium 2-(2-pyridyl)phenolate (hereinafter abbreviated to Lipp), lithium 2-(2′,2″-bipyridine-6′-yl)phenolate (hereinafter abbreviated to Libpp), etc., and zinc oxides (ZnO).

As for Cs2CO3, it is known that Cs metal is caused by deposition heat and an alcohol-based solvent to separate and functions as an n-dopant, so that an electron injection barrier is lowered and a good electron injection property is demonstrated in either the deposition method or the coating method.
Further, Patent Document 1 describes that provision of a predetermined aryl compound having a PO group and Cs ion or Ca ion dissolved in alcohol at a predetermined rate can improve an electron injection performance and an electron transport performance.
On the one hand, with respect to ZnO, an example has been reported in which a metal oxide, such as ZnO, TiO2, etc. being stable and conductive in the atmosphere is applied to the electron injection layer. According to this, a precursor of the above-mentioned metal oxide is spray-coated, then sintered at a high temperature (around 400 to 500° C.) for a long time (around several hours) to generate an oxide on the ITO substrate. However, the method of employing such a high temperature sintering process causes the organic layer to change and decompose, and therefore is difficult to apply to film formation on the organic layer and limited to an inverted type element structure.
On the other hand, with respect to film forming by a coating method without needing a high temperature sintering process, Patent Document 2 describes that, use of an organic-inorganic hybrid material in which ZnO particles and a predetermined aryl compound having a PO group are hybridized can improve the electron injection performance and the electron transport performance without using an alkaline metal, an alkaline-earth metal, and a compound thereof.